The use of ultrasound for destroying predetermined groups of tissue cells such as tumors has been known in the art for some time. After identifying the location of cells to be destroyed in a host body, ultrasonic energy is focused on the cells to be destroyed at a level sufficient to effect tissue destruction by thermal heating. Such a technique possesses the disadvantage of potentially overheating and destroying healthy cells surrounding the cells to be destroyed since precise, selective focusing is not always possible. Moreover, such a method is useless when the target cells to be destroyed are transient, such as in the case of leukemia.
Other ultrasonic techniques involve the direct application of ultrasound to a target area by means of a vibrating surgical instrument, for example in the form of a probe. In use, such a vibrating instrument must be surgically inserted into the patient for direct application to the target cells, whereupon the target cells are destroyed by direct application of intense mechanical vibration. Such a technique also possesses the disadvantage of high risk of damage to surrounding healthy tissue, especially when the target area is small. Moreover, such a device possesses the inherent risks of surgery, including risks of infection, reaction to anaesthetic, and collateral damage and bleeding during insertion.
There has been proposed a method for destroying selected cells without damage to nonselected cells through the application of ultrasonic energy which relies on the principal of resonance of the target cells for the target cell's destruction. U.S. Pat. No. 4,315,514 teaches the selective destruction of cells by first determining the resonant frequency of the cell as a whole to be destroyed, followed by the application of ultrasonic energy at sufficient levels to cause the entire cell to resonate at its resonant frequency with sufficient energy to ensure destruction of the target cell. Such a technique bears the inherent disadvantage of a lack of assurance of destruction of all selected cells. Specifically, a selected cell's resonant frequency is determined by taking a biopsy of the selected cells, and bombarding them with ultrasonic energy over a frequency range of 0.1 megahertz (MHz) to 4 MHz. The biopsy is interferometrically monitored for motion. The resonant frequency of the cells is determined to be that frequency which causes the greatest degree of movement in the biopsy of cells. However, since the resonant frequency of cells within a selected cell type to be destroyed can vary depending, for example, on its location within surrounding tissue, the location of the nucleus within the cell, or the location of other intracellular components within the cell, the determination of a resonant frequency which causes the greatest movement in the biopsy does not necessarily ensure that the resonant frequency of all of the cancerous cells to be destroyed will be identified.
It would thus be greatly advantageous for a method to be devised which could ensure destruction of selected cells within a host body without the need for invasive surgery and with a higher degree of assurance that all cells selected for destruction will be in fact destroyed.